The polyphenylene ethers are known and described in numerous publications, including Hay U.S. Pat. Nos. 3,306,874 and 3,306,875; and Stamatoff U.S. Pat. Nos. 3,257,357 and 3,257,358, all incorporated herein by reference. The high molecular weight polyphenylene ethers are high performance engineering thermoplastics having relatively high melt viscosities and softening points, i.e., in excess of 275.degree. C. They are useful for many commercial applications requiring high temperature resistance and can be formed into films, fibers and molded articles.
While possessing the above described desirable properties it is also known that certain of the properties of the polyphenylene ethers are undesirable for some commercial uses. For example, parts molded from polyphenylene ethers are somewhat brittle due to poor impact strength. In addition, the relatively high melt viscosities and softening points are considered a disadvantage for many uses. Films and fibers can be formed from polyphenylene ethers on a commercial scale using solution techniques, but melt processing is commercially unattractive because of the required high temperature needed to soften the polymer and the problems associated therewith such as instability and discoloration. Such techniques also require specially designed process equipment to operate at elevated temperatures. Molded articles can be formed by melt processing techniques, but, again, the high temperatures required are undesirable.
It is known in the art that the properties of the polyphenylene ethers can be materially altered by forming compositions with other polymers. For example, Finholt, U.S. Pat. No. 3,379,792 discloses that flow properties of polyphenylene ethers are improved by preparing a composition thereof with from about 0.1 to 25 parts by weight of a polyamide. In Gowan, U.S. Pat. Nos. 3,361,851, polyphenylene ethers are formed into compositions with polyolefins to improve impact strength and resistance to aggressive solvents. In Cizek, U.S. Pat. No. 3,383,435, incorporated herein by reference, Fox, U.S. Pat. No. 3,356,761, and Bostick et al, French Pat. No. 1,586,729, there are provided means to simultaneously improve the melt processability of the polyphenylene ethers and upgrade many properties of polystyrene resins. These patents disclose that polyphenylene ethers and vinyl materials, e.g., blended or grafted polystyrene resins, including many modified polystyrenes, are combinable in all proportions to provide compositions having many properties improved over those of either of the components. This invention provides compositions of the type disclosed broadly in such prior art, but with unexpectedly high impact strength.
Preferred embodiments of teh Cizek patent are compositions comprising a rubber modified high-impact polystyrene and a poly(2,6-dialkyl-1,4-phenylene)ether. Such compositions are important commercially because they provide both an improvement in the melt processability of the polyphenylene ether and an improvement in the impact resistance of parts molded from the compositions. Furthermore, such compositions of the polyphenylene ether and the rubber modified high-impact polystyrene may be custom formulated to provide pre-determined properties ranging between those of teh polystyrene resin and those of the polyphenylene ether by controlling the ratio of the two polymers. The reason for this is that the Cizek compositions exhibit a single set of thermodynamic properties rather than the two distinct sets of properties, i.e., one for each of the components of the composition, as is typical with compositions or blends of the prior art.
The preferred embodiment of the Cizek patent is disclosed to comprise poly(2,6-dimethyl-1,4-phenylene)ether and a rubber modified high-impact polystyrene (identified in Example 7 as Lustrex HT88-1 of Monsanto Chemical Company). It is known in the art that Monsanto HT-88 high impact polystyrene contains an elastomeric gel phase dispersed through a polystyrene matrix and that the particle size of the dispersed elastomer ranges from 2 to 10 microns with an average of 4 to 6 microns. This is shown, for example, in the photomicrograph in Vol. 19, Encyclopedia of Chemical Technology, 2nd Edition, 1969, pages 94, FIG. 2(b). Thus the preferred embodiment of the Cizek patent, which was disclosed to have a notched Izod impact strength ranging from 1.05 to 1.5 ft.-lbs./in. notch (Standard Method, ASTM-D-256) comprised a polyphenylene ether and a rubber modified high-impact polystyrene resin having a dispersed elastomeric gel phase with average particle size of about 4 to 6 microns.
It is generally recognized that the properties of impact resistant polystyrene are highly dependent on the number, size and character of dispersed elastomeric particles. Moreover, there is an optimum particle size in the region of 2 to 5 or 10 microns for rubber-modified impact-resistant polystyrene with a relatively narrow size distribution within this range. See, for example, Vol. 13, Encyclopedia of Polymer Science and Technology, 1970, page 392. In Duck et al, U.K. Pat. No. 1,127,820, it is disclosed that if the elastomer is dispersed in small particles of up to 1 micron, the impact properties of the rubber modified polystyrene are poor, being little better than crystal polystyrene. It is disclosed therein that optimum diameter will be 5 to 10 microns for good impact properties commensurate with good surface gloss. In Walker et al, U.K. Pat. No. 1,174,214, it is disclosed that uniformly small elastomeric particles, for example, those having a diameter of 1-2 microns, have an adverse effect on energy absorption characteristics of rubber modified polystyrenes. Such compositions are reported to have low elongation value and a low notched Izod impact strength. Walker et al teach the need to add particles with 5 to 25 micron size to overcome the adverse effect of the small, 1 to 3 micron, particles.
In the present state of the art, therefore, compositions of polyphenylene ethers and rubber modified styrene resins are known, in which the dispersed elastomeric phase has a particle size of about 4 to 6 microns, and which have a notched Izod impact strength of from about 1.05 to 1.5 ft.-lbs/in. notch. It is also known that the optimum elastomeric particle size in rubber modified polystyrene ranges from about 2 to about 5 or 10 microns, and that if the particle size is up to 1 or from 1 to 2 microns, the impact properties are poor, and only slightly better than crystal polystyrene.
In view of the above, it has now unexpectedly been found that compositions of a polyphenylene ether with a styrene resin and a rubber or with a rubber modified polystyrene resin can be provided with substantially improved impact strengths if the dispersed elastomeric phase is maintained below an average maximum of about 2 microns. The impact strengths are substantially higher than those of comparable compositions wherein the average particle size is increased above about 2 to, for example, about 6 microns, i.e., within the range disclosed in the prior art to be optimum. In addition, the surface appearance, especially gloss, is unexpectedly improved, as is the resistance to aggressive solvents, such as gasoline.